Change oil and drive it once so it gets up to running temp
Stop engine for the last time by cutting off fuel supply then let it starve
Take out engine and put it in a place as dry as possible
Be careful when you take it out that you don't spill coolant into any opening in the engine
If it's stored in less than 55% humidity, there is no
danger of rust, but the Mini engine is more forgiving to prolonged storage
than others, and if you can't ensure dry air, at least avoid humidity.
In my (sore) experience, the camshaft is the one that
goes after a long humid storage. Just the tiniest amount of rust here, and
you'll
be left with just a shaft few miles after starting to use it again. Then you go
no further.
To remove dirt from inside wheel arches use a high pressure steam washer. Let
dry *thoroughly*, at least a couple of dry warm days. Then follow up with thin
rust protecting oil. Then again with a moderate layer of underbody wax sprayed
on.